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Q8.18(iii):
Give plausible explanation for each of the following: (iii) During the preparation of esters from a carboxylic acid and an alcohol in the presence of an acid catalyst, the water or the ester should be removed as soon as it is formed.

Solution :

Theoretical Basis: Fischer Esterification Equilibrium

The preparation of an ester from a carboxylic acid and an alcohol is a reversible chemical process known as Fischer Esterification. The chemical equation for the reaction is as follows:

$RCOOH + R'OH \xrightleftharpoons[H^+]{} RCOOR' + H_2O$

This reaction exists in a state of dynamic equilibrium where the rate of the forward reaction (esterification) is equal to the rate of the backward reaction (hydrolysis of the ester) [Per the Law of Mass Action].

Step 1: Application of Le Chatelier’s Principle

According to Le Chatelier’s Principle, if a dynamic equilibrium is disturbed by changing the conditions (concentration, pressure, or temperature), the position of equilibrium shifts to counteract the change.

In this system, the equilibrium constant ($K_c$) is defined as:

$K_c = \frac{[RCOOR'][H_2O]}{[RCOOH][R'OH]}$

If the concentration of the products ($RCOOR'$ or $H_2O$) is reduced, the quotient of the expression becomes smaller than $K_c$. To restore equilibrium, the system must increase the concentration of the products by favoring the forward reaction [By the Principle of Dynamic Equilibrium].

Step 2: Mechanistic Necessity for Product Removal

By removing the water or the ester as soon as it is formed, the chemical system is prevented from reaching a state of equilibrium where significant amounts of reactants remain. The removal acts as a persistent stress on the system, forcing the reaction to continuously produce more ester to satisfy the equilibrium constant. This is a common industrial technique to maximize the Percentage Yield of the desired product.

Visual Representation of Equilibrium Displacement

Reactants Products Remove Water/Ester here → Shifts Equilibrium Right

Conclusion

The removal of products effectively prevents the reverse hydrolysis reaction ($RCOOR' + H_2O \rightarrow RCOOH + R'OH$) from occurring. Consequently, the forward reaction proceeds toward completion, significantly increasing the conversion efficiency of the alcohol and acid into the ester.

Final Solution: The preparation of esters is a reversible equilibrium process. By removing the water or ester as it forms, the concentration of the products is decreased, which—in accordance with Le Chatelier’s Principle—shifts the equilibrium toward the forward direction, thereby increasing the yield of the ester.


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